1. Field of the Invention
The following disclosure relates generally to thermoelectrics configured from heterostructures or thin layers of thermoelectric material to improve performance or usability of such thermoelectrics.
2. Description of the Related Art
The bulk properties of thermoelectric (TE) materials can be altered if the materials are formed from very thin films or segments of alternating materials. The resultant assemblies formed of segments of such thin films are usually called heterostructures. Each film segment is the order of tens to hundreds of angstroms thick. Since each segment is very thin, multiple segments are needed to fabricate cooling, heating and power generating devices. The shape, dimensions and other geometrical characteristics of conventional heterostructures often make attachment of suitable thermal heat transfer members and electrodes to the individual heterostructures assembly difficult. Further complications arise in the extraction of thermal power from the structures. New fabrication techniques, material combinations, and forming methods are required to fabricate TE elements from such materials. New fabrication techniques are even more critical for systems made from thousands of segments since materials formed of many segments tend to be fragile and weakened by (1) internal stresses that result from fabrication, (2) the very nature of the materials and (3) internal weakness caused by contamination and process variation. Further, certain TE materials, such as those based on Bismuth/Tellurium/Selenium mixtures, are inherently mechanically weak and hence, fragile in heterostructure form.
Heterostructure TE materials generally are configured to be long in one dimension (e.g., wires) or two dimensions (e.g., plates). The TE materials are usually anisotropic with varying thermal, electrical, and mechanical properties along different axes. Electric current either flows parallel to a long dimension or perpendicular to the long dimension(s). In TE elements where the current flows parallel to the long dimension, the length can range up to thousands of times the thickness or diameter of the material. To achieve the desired performance, such TE elements can be made of a multiplicity of heterostructure wires or plates.
Various embodiments using heterostructures in forming thermoelectric elements are disclosed. The heterostructures are constructed with layers of bonding and/or intermediate materials that add strength and/or improve manufacturability of completed thermoelectric elements formed of the heterostructures. In addition, the bonding and intermediate materials are used in various manners to facilitate or enhance the operation of thermoelectric assemblies. The thickness of the intermediate and bonding materials take into account the desired thermal and electrical characteristics and attributes for the particular configuration or application. Both the thermal conductivity and thermal conductance can be taken into account, in considering the thickness of each bonding and intermediate material.
Several configurations for thermoelectrics are described. One configuration involves a thermoelectric element that has at least two heterostructure thermoelectric portions of the same conductivity type (such as N-type or P-type). It should be noted that the use of the term xe2x80x9csame conductivity typexe2x80x9d in this configuration does not mean that these portions need to be of the same material, nor doping concentration. An electrically conductive material is coupled to the thermoelectric portions to form at least one electrode.
Preferably, the heterostructure thermoelectric portions form layers in the thermoelectric element, and the electrically conductive material is coupled to at least one of the layers at at least one end of the layers. Preferably, the conductive material couples to all or substantially all of the layers, where the electrode is an end electrode. Alternatively, the electrically conductive material may be coupled to at least the top or bottom of the layers.
In one configuration, the heterostructure thermoelectric portions form wires or a wire bundle, and the electrically conductive material forms at least one electrode at the end of the wire bundle. Preferably, an electrode is provided for each. Alternatively, the electrically conductive material is coupled to at least the top or bottom of the wires, or separate electrodes are provided for the top and bottom of the wires.
In one example, a bonding material is between the at least two heterostructure thermoelectric portions. The bonding material is advantageously configured to reduce the power density or the shear stress in the element, or both.
An intermediate material may also be provided between the bet heterostructure thermoelectric portions and respective electrodes. Advantageously, the intermediate material is configured to reduce shear stress in the heterostructure thermoelectric portions when the thermoelectric element is operated. For example, the intermediate material may be resilient.
In one example, the heterostructure thermoelectric portions are of substantially the same thermoelectric material. The heterostructure thermoelectric portions may also be constructed of at least two layers of heterostructure thermoelectric material.
Another example of a thermoelectric element described has at least two layers of substantially the same thermoelectric material of the same conductivity type. At least one electrically conductive material is coupled to the thermoelectric material to form at least one electrode. In one form, the electrically conductive material is coupled to the layers at at least one end of the layers. Preferably, an electrode is provided at at least two ends. Alternatively, the electrically conductive material is coupled to at least the top or bottom of the layers, forming top and bottom electrodes. The layers may also form wires, with the electrodes coupled to the wires at at least one end of the wires, or coupled to at least the top or bottom of the wires.
In this example, a bonding material may also be provided between the at least two layers. Advantageously, the layers and the bonding material are configured to reduce the power density of the thermoelectric. The layers and the bonding material may be configured to reduce shear stress as an alternative, or in addition to, reducing the power density.
An intermediate material may also be provided between at least one electrode and at least one layer of the thermoelectric material. Preferably, the intermediate material is also configured to reduce shear stress in the layers. In one configuration, the intermediate material is resilient.
The at least two layers may also be heterostructures, as with the previous example. The heterostructures themselves may be made from at least two layers of heterostructure thermoelectric material.
Also disclosed is a method of producing a thermoelectric device involving the steps of layering at least two heterostructure thermoelectric segments, and connecting at least one electrode to the segments to form at least one thermoelectric element.
The step of layering may comprise bonding the at least two heterostructure thermoelectric segments with a bonding material. A further step of providing an intermediate material between at least one of the at least two heterostructure thermoelectric segments and at least one electrode may be used.
Preferably, the layers and/or the bonding and/or intermediate materials are configured to decrease power density. One or another, or all, may be configured to reduce shear stress as well, and/or reducing the power density.
Another method of producing a thermoelectric device involves the steps of forming at least two layers of substantially the same thermoelectric material, and connecting at least one electrode to at least one of the layers. Preferably, the step of forming involves bonding the at least two layers with a bonding material.
Advantageously, as mentioned above, the bonding material is configured to decrease power density and/or shear stresses. Similarly, an intermediate layer may be provided between the layers and the electrodes.